1. Field of the Invention
The present invention relates to an apparatus for synthesizing diamond films by a Direct Current Plasma Assisted Chemical Vapor Deposition (DC PACVD), and in particular to an apparatus for increasing the deposition area to several inches in diameter by using multiple cathodes (Multi-cathode DC PACVD). The present invention overcomes the problem of non-uniformity of diamond films and instability in the system of the U.S. Pat. No. 5,647,964.
2. Description of the Background Art
Since it was reported in the 1980""s that diamonds could be deposited by a CVD method, there have been many studies to develop a new method for diamond deposition. Some methods, such as a hot filament CVD, high power microwave(MW) PACVD and DC arc-jet processes have shown excellent performance in diamond deposition, and are currently used in the industry. The hot filament CVD method with simple structure is possible to deposit diamond films on a large area, about ten inches in diameter. However, diamond films contains a lot of non-diamond phases, and the growth rate is less than few xcexcm/h. Thus, this method in general is used to deposit diamond films of tool grade(lowest grade in CVD diamonds). While, other two processes can deposit high quality diamond films, which are applicable to thermal management substrates(thermal grade: medium grade) or window materials(optical grade: highest grade). They can deposit diamond films on a large area above 8xe2x80x3 in diameter at a growth rate higher than that by the hot filament CVD method.
Nevertheless, the cost of diamond films is still high, due to the high cost in equipments and operations.
On the other hand, the DC PACVD method is expected to be the most economical one among all other CVD processes for depositing thermal or optical grade diamond films, because the equipment is very simple with small gas consumption, and generates near thermal plasma which enables deposition of high quality diamond films at a growth rate equivalent to that by the DC arc jet process, However, it had some critical problems of instability of the plasma and a small deposition area around 1 inch in diameter.
We had stabilized DC plasma by using a hot cathode of which temperature was maintained higher than 2000xc2x0 C. to prohibit the deposition of the solid carbon on the cathode surface, that was a main cause of an arc generation which shut down the DC glow discharge, in U.S. Pat. No. 5,647,964. In this patent, we also had showed a concept that DC plasma could be expanded by using multiple cathodes(multi-cathode DC PACVD), which would enlarge the deposition area.
However, diamond films deposited by the multi-cathode DC PACVD apparatus using three cathodes(three-cathode structure) on a substrate with 4 inches in diameter was not uniform. The uniformity of the film thickness is higher than 50%. Here, the uniformity of the film thickness may be computed by the following Equation.   Uniformity  =                              Max          ⁢                      xe2x80x83                    ⁢          film          ⁢                      xe2x80x83                    ⁢          thickness          ⁢                      xe2x80x83                    ⁢                      (                          T              max                        )                          -                  Min          ⁢                      xe2x80x83                    ⁢          film          ⁢                      xe2x80x83                    ⁢          thickness          ⁢                      xe2x80x83                    ⁢                      (                          T              min                        )                                      Max        ⁢                  xe2x80x83                ⁢        film        ⁢                  xe2x80x83                ⁢        thickness        ⁢                  xe2x80x83                ⁢                  (                      T            max                    )                      xc3x97    100  
Such non-uniformity of the films was due to the non-uniformity of the plasma generated in the three-cathode structure. We observed that the plasma was not uniform, but was nearly independent of each other. Therefore, in the multi-cathode DC PACVD method, the uniformity of the plasma, affecting directly the uniformity of diamond films, is very dependent on the inter-cathode distance. Thus, in order to deposit a uniform film on a deposition area, the inter-cathode distance has to be maintained within a certain range.
In addition, in the multi-cathode DC PACVD apparatus, using a disc-shaped cathode as shown in U.S. Pat. No. 5,647,964, does not guarantee its stability. In the apparatus, plasmas formed under each cathode shifts to the central direction and this gives rise to a temperature difference on each cathode; lower temperature at outer zone than that of the inside. In this case, a solid carbon deposits easily on the lower temperature zone of the cathode, especially at a high methane concentration over 8%. Also the temperature gradient of the cathode causes a concentration of the electric field at the high temperature zone of the cathode, and may be easily transformed to arc.
Accordingly, it is an object of the present invention to provide an apparatus for synthesizing a diamond film by a multi-cathode DC PACVD which is capable of maximizing the uniformity of crystallinity and thickness of diamond films on a certain deposition area, and minimizing the temperature difference in each cathode for thereby removing instability of the apparatus and implementing good reliability of the apparatus.
To achieve the above objects, there is provided an apparatus for synthesizing a diamond film by the multi-cathode DC PACVD wherein the number of the cathodes is seven, and six cathodes are radially arranged at an angle of 60xc2x0 about one central cathode, so that a basic array of the cathode is a regular triangle in which the inter-cathode distance is variable with a diameter of the substrate. Also, the shape of the six cathodes disposed at edge is rod of which the lateral face is joined to a connecting rod, so as non-uniformity of the temperature in the cathode to minimize.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.